I have received an overall of 6 bands in IELTS exam. Am I eligible to study Diploma in New Zealand?

Start by looking at which context you can use the words in. You can establish associations between new words and familiar concepts. To aid memorisation, practice actively by using the words in daily life.

The speaking test lasts around 11-14 minutes. The test includes 3 parts - Introduction & Interview (Part 1); Cue cards (Part 2); Follow-up discussion (Part 3).

 Yes, vocabulary is very important as it conveys a deep understanding of the language. Also, vocabulary (Lexical Resource) marks 25% of your overall band score of IELTS Speaking.

Yes, idioms can aid in improving your score for lexical resource, i.e., one of the assessment criteria you will be assessed on. You can use idioms to show your ability to use language naturally and appropriately. 

No fixed number of idioms to be used in the IELTS exam exists. However, make sure to use them in right context. Use idioms in your sentences naturally and moderately. Aim to use 1-2 idioms on a relevant topic throughout your responses.